Insulation of transfer lines and storage vessels for cryogens and other temperature-sensitive materials is required to prevent transfer and storage losses, and to reduce other economic losses such as maintenance and replacement costs.
Currently transfer line insulation can be grouped into three general categories: vacuum-jacketed multilayer insulation (MLI), powders (e.g., perlite), and solid foams, (e.g., polyurethane). Vacuum-jacketed lines with MLI have the lowest heat leak; however, they have high initial cost and require periodic vacuum maintenance. Evacuated powder insulation is less expensive, but has lower thermal performance, and therefore is not generally used for transfer line insulation. Polyurethane foam insulation is used on many large cryogenic transfer lines because of its low initial cost and relative ease of installation. However, it has a much higher heat leak than evacuated MLI and is subject to severe degradation from repeated thermal cycling and environmental exposure.
Foam insulation degradation is often associated with cracking that expose portions of the cold surface to the ambient air. This causes oxygen-enriched air to form, creating a severe fire hazard. In the case of liquid hydrogen lines, liquid air, rich in liquid oxygen, will stream off the exposed surfaces. In addition, foams are not oxygen compatible and can burn in an oxygen rich environment. For these reasons, foam insulation should be avoided where personnel are working, and is unsuitable for manned space applications such as reusable launch vehicles.
The fabrication of microsphere particles from various components is known (see, for example, U.S. Pat. Nos. 4,459,145; 4,415,512; 4,303,732; 4,303,730; United States application 2002/0004111; European Patent application EP 1172341). U.S. Pat. No. 3,769,770 (Nov. 6, 1973) describes an arrangement of spheres individually coated with a material such as nickel for use as a thermal barrier for extremely high temperature applications. U.S. Pat. No. 5,713,974 (Feb. 3, 1998) describes the use of evacuated microspheres in an insulating coating such as an elastomeric roof coating. Parmley investigated the use of 80 μm diameter hollow borosilicate glass spheres at a pressure of 10−5 mm Hg as an insulator of a stainless steel jacket hemisphere formed from a flat piece of stainless steel (Parmley, Microsphere Insulation Systems, Applications of Cryogenic Technology Vol. 8, Oct. 14, 1975, pp. 161-167). This system was found to be too expensive.
An insulation system that combines ruggedness, low mass, ease of application (including field installation), good thermal performance, low life-cycle cost, safety, and little or no maintenance is needed. The current technologies are either expensive (evacuated MLI) or degrade quickly (polyurethane foam). In addition, current technologies are maintenance intensive to retain the original performance level.